This invention relates to a high-speed beamformer for hydrophone or antenna arrays having symmetry under rotation, providing either a time-domain or a frequency-domain output.
Beamformers in the prior art usually fall into one of two categories: Time-domain beamformers using either delay lines or simulating delay lines in computer memory, or frequency-domain beamformers using the Fourier transform, where the Fourier transform is usually calculated using the fast Fourier transform (FFT) or the chirp-Z transform.
Time-domain beamformers using delay lines to form simultaneous multiple beams have the disadvantage that they require many output taps per delay line, since a summation across the delay lines is needed for each beam output. This makes such devices difficult and expensive to build because many interconnections are required. Time-domain beamformers using a computer memory to simulate delay lines require a vast number of memory accesses, and hence are quite limited in speed.
Prior art frequency-domain beam formers have the disadvantages that they either depend upon matrix multiplication, which is quite slow, especially if many beams are needed at many frequencies; or require a multi-dimensional Fourier transform to form beams for a line or planar array with uniform spacing. The latter beamformers are not only limited to the uniformly spaced rectangular grid, but also require interpolation if a time-domain output is desired.
Another class of beamformer which can handle arbitrary geometries has been proposed using triple product convolutions, but such devices are presently extremely limited in dynamic range because of the difficulty of performing a threefold multiplication in analog form.
Background information which would be useful for understanding this invention may be found in U.S. Pat. No. 4,060,850 entitled BEAM FORMER USING BESSEL SEQUENCES, which issued on Nov. 29, 1977, to Jeffrey M. Speiser. The invention therein described, however, is limited to arrays whose elements lie in a single circle. The present invention provides an extension to a three-dimensional array having circular symmetry, as well as explicitly providing for focusing and vertical depression or elevation of the steering directions if desired.